A position-sensitive ionization chamber for thermal neutrons

A position-sensitive neutron detector based on He-3 is being developed for a national crystal backscattering spectrometer (CBS) at a pulsed neutron so urce. It comprises a single gas volume, of constant depth, in the form of a n annulus that is also part of a truncated cone. Charge from each event is collected on one or two anode strips, out of a total 336 around the annulus , yielding an angular resolution of about 1 degrees. Low-noise signal proce ssing, in the form of a monolithic preamplifier/shaping amplifier connected to each anode strip, provides electronic noise that is small enough to per mit detector operation in ionization mode. In addition, the segmented anode s ensure near complete induced charge collection. This new detector therefo re has significant advantages over previous detectors used in CBS instrumen ts, which primarily consisted of arrays of individual He-3 filled cylindric al counters with dead regions and nonuniform detection efficiency. The opti mization of gas mixture/pressure, drift field, and shaping. time is signifi cant in achieving successful performance in the new detector.